In the prior art, aluminum alloys are the alloys of choice for heat exchanger applications. These alloys are selected for their desirable combination of strength, low weight, good thermal and electrical conductivity, brazeability, corrosion resistance, and formability.
Typical applications include automotive heater cores, radiators, evaporators, condensers, charge air coolers, and transmission/engine oil coolers. One particular application that requires a good combination of properties is fin stock for radiators. In these applications, the fin stock is arranged between stacked tubing that carries the radiator cooling media. The tubing is situated between headers which redirect the cooling media flow between layers of tubing and which also can contain the radiator inlets and outlets. Typically, the tubes are clad with a brazing material and the entire assembly is brazed together in a controlled atmosphere braze (CAB) process using a brazing flux.
The trend in the heat exchanger industry is to continually downsize components. This downsizing requires new demands on the heat exchanger materials in terms of properties and performance capabilities. Such demands are recognized in U.S. Pat. No. 5,217,547 to Ishikawa et al. This patent notes the desire for making fin stock of thinner gauge to meet the demands of higher performance and increased compactness for heat exchangers. While thinner gauge may result in lighter weight heat exchangers it can also result in lower strength, particularly as a result of the brazing process wherein the heat exchanger may see temperatures of 600.degree. C. These elevated temperatures can cause sagging during brazing, loss of fin integrity, and an unacceptable brazed product.
Ishikawa et al. attempt to overcome this problem by exotic alloying of an aluminum alloy typically used in fin stock application, i.e., AA3003. To provide high temperature deformation and sagging resistance, Ishikawa et al. teach an aluminum alloy for fin stock with particular levels of iron, silicon, zirconium, zinc, tin, and indium. The drawback to these remedies is that often times these exotic alloys can be difficult to manufacture, thereby greatly increasing the cost of the fin stock material.
Another apparent solution to the problem of decreasing the thickness of fin stock material is to employ a higher strength aluminum alloy material. While employing a higher strength material may offer benefits in terms of increased sagging and high temperature deformation resistance, these higher strength materials are problematic in the brazing sequence of heat exchanger manufacture.
An often-used brazing process for heat exchanger manufacture is the CAB process employing a Nocolok.RTM. flux. This flux is a non-corrosive flux made up of a mixture of potassium and fluoro-aluminates. The flux functions at brazing temperatures by melting, spreading and dissolving the oxide film. Certain alloying elements often found in higher strength aluminum alloys, e.g., magnesium, can be detrimental to the brazing process, including adversely interacting with the fluxes used during brazing. Consequently, while employing a higher strength aluminum alloy may solve the sagging problem during brazing, other problems may crop up due to the incompatibility between the alloy itself and the brazing process.
Another problem in the prior art is the need for heat exchanger materials, e.g., fin stock, to have certain levels of post-braze tensile strength and electrical conductivity to meet customer's specifications. It is difficult to meet specifications that combine both higher levels of strength and electrical conductivity with presently available materials at reasonable costs. Aluminum alloys having high strength will have lower electrical conductivities and cannot meet such specifications. Similarly, aluminum alloys with high electrical conductivities will have insufficient strength to meet such a specification. While exotic alloys may offer a limited solution to this dilemma, costs to make such alloys make this solution undesirable for many manufacturers.
Accordingly, a need has developed to provide an improved material for heat exchanger use that overcomes the drawbacks to the prior art solutions noted above. The present invention solves this need by providing an aluminum alloy composite material made up of a high electrical conductivity cladding layer and a lower electrical conductivity core. The composite material exhibits high post-braze strength, improved post-braze electrical/thermal conductivity, excellent corrosion resistance, and brazeability in brazing processes such as CAB. The material is ideal as bare fin stock to be brazed to braze clad tubing for heat exchanger applications.
The use of composite materials made of aluminum has found application in cable shielding. U.S. Pat. No. 4,010,315 to Mildner discloses a cable shielding tape, which has a first layer of substantially pure aluminum bonded to a second layer of an aluminum alloy. While Mildner may disclose a composite material, this patent is not concerned with the brazing problems faced by the prior art nor does this patent suggest any type of a solution to such problems. Other composite materials have been proposed as in U.S. Pat. No. 4,146,164 to Anderson and U.S. Pat. No. 5,011,547 to Fujimoto et al. However, these patents are primarily directed at braze clad materials, not bare fin stock material and the like as is the present invention.